Abdelhamid Abaaoud, who French authorities say is the mastermind behind the attacks, had been identified as an accomplice of two jihadis who were killed in a shootout in Verviers, Belgium in January.
Photograph: AFP/Getty Images

With every new piece of information that has come to light about the perpetrators of the Paris attacks, it has become clearer that the intelligence services in France and Belgium knew about their jihadi backgrounds.

Several had dossiers identifying them as radicals. At least five had travelled to fight in Syria and returned to homes in France or Belgium. They were dots on the radar screen but the security services failed to join them up and so overlooked the gathering conspiracy.

“What we know is that most of these people came back from Syria and nobody stopped them,” said Natalie Goulet, a member of the French senate foreign and defence committee. “Whatever the reform that has been implemented [in the intelligence agencies] it’s not working.”

In Belgium, the parliamentary committee with oversight of the country’s intelligence services has opened an inquiry into the failings in the run-up to the attacks. A Green MP, Stefaan Van Hecke said: “It appears that the terrorists managed to evade the intelligence radar and the police. The question is to know what more we can do now.”

Abdelhamid Abaaoud, who French authorities said on Monday was the mastermind behind the attack, had been identified as an accomplice of two jihadis who were killed in a shootout at a house in the eastern Belgian town of Verviers in January. By his own account, they had travelled back from Syria with him the previous year and set up a safe house there.

After the raid on the house, the authorities “figured out that I had been with the brothers and that we had been planning operations together”, he said. “My name and picture were all over the news yet I was able to stay in their homeland, plan operations against them, and leave safely when doing so became necessary.”

At one point, Abu Oud recounts being stopped by a policeman who checked him against the picture in his wanted notice but still failed to recognise him and let him go.

One of the attackers at the Stade de France, Omar Ismaïl Mostefai, had a French police “S” file, denoted suspected radicalisation, since 2010. He had gone to Syria in 2013 and returned to France in the spring of 2014. Turkish authorities claim they twice tried to alert their French counterparts to the threat he represented, in December 2014 and June this year, but the warnings do not appear to have prompted action.

Similarly, Sami Amimour, one of the gunmen at the Bataclan, had been detained in October 2012 on suspicion of terrorist links, and had an international arrest warrant out on him after he broke his parole the following year and travelled to Syria. Yet he returned in mid-October 2014, and was able to remain at large until the attacks.

In another embarrassment, Salah Abdeslam, who hired one of the cars used by the attackers and is the brother of one of the terrorists who blew himself up outside the Comptoir Voltaire cafe, was stopped in a vehicle with two other men on the French-Belgian border a few hours after the attack and questioned, but then released.

“That represents huge negligence, if that is confirmed,” said a former senior MI6 official. “It has to be asked why a large scale attack like this, which would have a footprint, was not picked up by the intelligence services.”

Speaking in Washington on Monday, the CIA director, John Brennan, blamed the intelligence gaps leading up to the Paris attacks on the increased ability of terrorist networks to communicate without being intercepted by the security services.

Brennan said there had been “a significant increase in the operational security of a number of these operatives and terrorist networks as they have gone to school on what it is that they need to do in order to keep their activities concealed from the authorities”.

The Belgian government has said that terrorist networks in Belgium had begun used the Sony Play Station 4 for its communications, as a way of avoiding surveillance.

Brennan blamed such developments on “a number of unauthorised disclosures and a lot of handwringing over the government’s role in the effort to try to uncover these terrorists”, and called for intelligence agencies to be given a freer hand in conducting surveillance.

However, François Heisbourg, a former member of a French presidential commission on defence and security, said the biggest problem was not a shortage of information about suspects but a lack of capacity to process that information.

“It is less a failure of intelligence than the ability to follow through on the intelligence data,” said Heisbourg, now chairman of the International Institute for Strategic Studies and the Geneva Centre for Security Studies. “The domestic security service was revamped in 2013 but it is still underfunded and undermanned. It is the process of being reformed but reform only produces fruit over four or five years.”